Integrated circuits for high speed applications frequently employ Schottky diodes because of their fast switching speed. A Schottky diode is formed by contact between a metal and a lightly doped semiconductor (typically silicon). When a forward bias voltage is applied across the diode, the potential barrier (Schottky barrier) between the metal and silicon is lowered and an electric field is established in the silicon. The majority carriers (electrons in N type or holes in P type silicon) to move in the direction of the field and create current flow. Since there is no stored charge in the Schottky diode, the forward current in the diode is extinguished very quickly when the bias voltage is reduced to zero or reversed. This allows the diode to switch on and off rapidly making it very attractive for high speed applications. Conventional pn junction diodes switching speed is typically on the order of about 100 ns. Schottky barrier diode switching speed is on the order of about 100 ps—three orders of magnitude faster.
Metals such as aluminum and copper and refractory metal silicides such as are commonly used in conventional polysilicon gate integrated circuits typically have too low a Schottky barrier height to form useful Schottky diodes. Typically, addition of useful Schottky diodes to a conventional integrated circuit requires a significant number of additional processing steps to deposit, pattern, and etch the Schottky metal. These extra processing steps add substantial cost to the process.